I i
I
S. II - 2.2
THE INFLIJENCE ON THE EITVIRONMENT OF COASTAL STRUCTURES
RECENTLY BUILT IN THÊ NETHERLA.IIDS
by
C, STIGTER, Bo6kali! hterrational BV, PostbE 4i], 3350 ÁÀ PaPéDdt4ht
H.J. VEREAOEN, MinistÍy of TraDspolt and Public Works, KoEitrsskade 4, 2596 AA Thê Haeuê H.J. DE vaIDND, D€Ift HydrÀulicÊ, P.O. Bor 177, 2600 MH Delft
R.W. VAN DER WEUDE, thê Port ofR tt€rdam, Postbus 6622, 3002 AP Rott€ldam C. VAN RAÀLTEN, Mitristry of TlaDspoÍt and Public Work!, KoDiDsskadè 4, 2596 AÁ The Itu8ue
M. LOXHAM, Soilmechuics Deift, Stieltj$weg 2, 2618 CK Dêlft {Th€ Netàêrland!)
Part I
THE IMPACT ON TIIE ENVIROIMENT OF COASTAL STRUCTT'RES IN GENERAI,
I. GEI\ÍERAL
I.I. IN'INODUCTION
In the Netherlands today much attention is paid to the effect of coastal structures on the €nvironment. In the past it ri/es usual to quantify the effect of such a structure on its environment solely with th€ purpose of deciding whether that effect could imply any hezard to the shucture itself (or to stluctul€s in the neighbourhood).
Nowadays environmental impact studies are mainiy car. ried out to see what possible chaDges the str_ucture may bring about in the integral coastal system. However, changes in the coastal system can only be studied if information is alEady available regarding that coastal system. T'herefore scientists and engineers from various disciplineE, such as hyclraulic engineering, sedimentology, geology, physical ocean" ography and Earine biologf have joined forces to desclibe and analFe the long-tèrm aad large-scale morphological evolution of thê Dutch coaslal zone as an i[tegrated system.
r.2. THREE ]IAJOR PROJECTS
The above approach has been used in three majoÍ ploJects, vrz i
- A pilot project to €váluate the effect of the big enclosur€ works in the southwestêm part of the Netherlandsi
- A programme focused on the coastal evolution of the Netherlands on historical and geological scales (Coastal G€n€sis) [De Vroeg et al., 1988], and
- A programme to provide information lor the Coastal Memorandum, in which the Dutch goveÍnmeÍrt presents the narional coastal policy to parliàment.
r.3. COASTAL POLICY
The pdncipies of the Dutch coastal policy lverhagen, 19891, as laid down in the Coastal Memorandum, are: (a) Rêsponsibility fo! the safety ot
"'o.' ,u".,og "u " "." defence) rests with local, specialized authoities (polder boaÍals). If erGion endangers the sàfety oí rvater retaining dunes, these authorities are authorized by law to demand action by the national gov€Ínment in oder to strengthen the dunes.
O) Du[es are preferably strengthened by beach nourishment. This also keeps the coastline at its present location. (c) If the coast is eroding and there is no danger of
inuddation of low Iying polders, but only the danger of . Ioss of dune area, a decísio[ on coasta] maintenance is made by the national government, depending on the value ilr the dunes (ecological, ecoDomical, etc.) and Iikewise on the costs of the works.
(d) Works to maintain the coastline lor the above-mentioned second reason are paid for mainly by the national government. However, the dêcision to pay for such works depenals also on the intention of local authorities to cooperate on a Íilancial basis.
(e) Works in the coastal zone with an aim other than sea defence o! erosion prevention require a coastal en, vironmeDtal irnpact statehent.
The philosophy behind this policy is that it is oun p me national aim to keep the sea alefences at the requircd level of safety. However, wê do not do this by suppressron of the unwanted processes (erosion is rlot stoppeil by building sea walts) but by guiding and controlting them. B). using an ilterdi8cipiinary, integrated
approach we can preveÈ trose processes from developrng jn an undesirable dtrecrion.
2. COASTÁL STRUCTL'RES RECENTLY BUILT
IN THE NETHERLANDS
2.1. THE DELTA PROJECT
Áfter the storrn surge disaster of 195g it was decided to close the major tidal inlets in th€ Netherlands. This plan, kno$n as the Deita project, was completeal in 1988. Figrre 1 shows the plan.
Because the current pattern in the tidal area in the aouthwest€rn part of the Netherlands was completely changeal by the Delta Project, the plan hail a significant influence on the coastal arca irl front of the tidal inlets.
THE OPEN EASTERN SCHELDT
During rhe execurion of lhe project it was decided not to close the Eastern Scheldt estuary by a dam, but to build a storm surge barrier. In this way the tidal inJluence in the estuary could remain, which had many ecological advantages. Because ol inland dams, there is no longer any inÍlux oí (polluted) íiesh water in this estuary. The water quality in the estuary has become very good, and this has resulted in an importsnt aquatic ecosystem.
LAKE GREVELINGEN ÀND LAKE VEERE
The other estuaries are now salt vratêr Iakes (I_ake Grevelingen and Lake Veere). The former sanilbanks have become islands, which EuÍIer fmm erosion. Many constnrc-tions have been designed and built to protect the sbor€line.
I r ( r t . . Í r r Í r r
lO dtt.on
1 ' t 6
Iisllr€ I - Thê Deltá project D€lreplan)
-This has been done in such a way as to enable the shoreline to achieve a high ecological value (see Sêction 3'2.).
2.2. SLIJI:|ER ON TIIE MAASWAKTE
For ei<pansion of the poÉ oí Rotterdam hore space wall required and this was not aváilable on the maiDland. So it was decided to build a new harboÈÍ comPleÍ expanding as a peninsula into the sea, the so-called Maasvlakte.
Dredging the harbour basins results itr huge quaDtitiês of mud. Most of tàis mud is polluted and cauot be dumped iBto thê sea, It ll|as not possible to find a location fo! a disposal area on the maiÍrland, therefore one was ilesigned in the íorm of a pemnsula attached to the Maasvlakte This plan is called thê n Slufter-plan ". It became operational in 1988. Figure 2 shows the peninsula Maaevlakte / Slufter'
FiEtrle 2 - Aêrial piciure oí Maasvlaktê/Sh.fter
2.3. MAN.IIADE ISIANDS
Thê nexi step in the development of sPecial storage aleas could be the creation of land even further out at sea
The idea of realizing land right oul at sea goes back as far as thê eeily seventies. The idea Yías bortr against the background of the Ecalcity of land in the densely populated delta region of the River Rhine.
Since then various projects have been developed, initiaily intended fo! the establishment of port and sea'bound activi-ties, but later on for more speciflc aPplications, such as thê storage of lightly contaminated alredged materials, en$gy supply, refuge harbours, etc. The main objective of all these projects rvas to create new large-scale areas relieving the pressure on the mainland in many ways and apt to being etrvironmentally well conholled.
TSo iEportant rêasolls for hesitation before deciding to build such ofrshorê islanals ale the co6t of theil realizátion and a po$ible consequent eduction in swsreness of the need to solve pollution problems st the source of Production.
In oldeÍ to arrive at acceptable cost levels, the water depth should not be too Sreat and the islands should be rather large (several kilometres iD diamet€r)' In other wor'b, such isla ls can be reasonably consialered' lf the storage capacity is large and the location of the island is chosen in shallow water.
Various investigations hsve been carned out, but it has not yet come to the actuáI realization of such a plojêct'
2.4. BEACII NOI,'RISIIMET{T
The sandy coast of the Netherland5 is eroding at several Iocations. The erosion is of the order of 0.5 - 5 D/year' Because the Netherlanab ale densely populated' any loss of land is always a difÊculty. But in the case of dune erosion the ploblems are even mote severe. At dany places the dunes are the sea defence for the low-lying polders behind theE, and this protective barrier is very thin, mostly only one row of dunes. Erosion endangers the sea defence and is theÍefore a direct threat to the suwival of the Netherlands. As the dunes . ale of imPortant ecological and recreational value, prctecting them is therefore of the gleatest signiíi_ cance in a densely poPulated country.
It has bêen decided at government level to call a halt to erosion. Elosion will always be stopped if there is any risk of inundation of poldeÉ due to a breakthrough in the dunes. In ail other cases. the Íinal dêcision to confiol erosion will depend on a cost-bênefit analysis of the functions in the endangeled dune area. On the ba.sis of this policy, 30 beach nourishment projects have been impiemetrted
3. ASPECTS
PARTICIJ'LARLY L\N/ESTIGATED
3.1. IIiFLUENCE OF TIIE DELTA PROJECT OI THE MARINE ECOLOGY
Closure of the northem tida.l inlets around 1970 meant that the velocity of inward and outwald flow seawsÍd of the clams was reduced by between 50 /. atà 90 r'o.
Erosion at the front of the deltas, which is caused by wave action and tidal cunents aDd which was formerly compensated for by sediment oliginating Íïom the tidal inlets, couid now continue unabated. As a resuit, the shape oí the ebb.delta began to chatrgê.
The eroded material Eettled iÍr the former tidal channels aud formed transve$e bars. Mud, particularly, was deposited in the channels near the dams, iD some places more than 0.50 m per yeÊr. The tratrsverse bars developed maialy along thê northêrn edge of the ebb-tidal deltas. They increase in height landward and also tend to move in thiÊ directioD.
Itr less than 10 yea$ these transversal bare developed illto lodg intertidal longshore bars lKohsiek, 1988]. Flood
Figlre 3 , Sandbad{€ dèvelopins in f.ont ot rhe Dui.h Coási
currents at high tide traBported sand northwards ov€I the banks, counterbalancing the wave sctivity. As a result, the level of the bals stabilized and the speed at which they moved landward was reduced (Figure i,.
InvestigatioÍrs airÍred at quantifying these and future deveiopments are in progïess. The recenrly complered srorm surge banier in the Eastern Scheldr has rcduced the tidal prism by 25 9.. This is expected to cause developments similar to those in the other (fully ciosed) tidal inlets, although these changes will be less extensrve.
Because of the geomolphological changes in the area, the nalulal envlronment is aÍïected at all levels. The new hydraulic conditions and water quality aÍfect the primary production of algae. Changes in bed level caused by Bhoal formatioll or erosion, siltation ill deeper channel sections, anal changes in bed composition due to the addition or removal of mud, greatly aíïect the development of benthic fauna. These areas, with their drying shoals and abundant food, fulíil an hportant function for many breeds of birds Í,,hich rnigrate over them or winter there. The erosion of shoals in thê Eastêrn Scheldt rcduces its capacity to suppod birds and, by contrast, the capacity of the Outer Delta is increased by the growth of banks aDd shallow area_s. As a result thele will be changes in the types and number of birds in the a!ea.
Large colonies of cockles have developed locally in the shelt€red areas behind the new sandbanks. The cockle harvest by the cohmerciÊl shellfish industry will increase. Mussels will also breed here. In the Eastem Scheldt, the emsion of shoals and the local increase in mud deposits will destroy some of the musÊel breeding areas. By conhast, however, the reductioD in current velocities enables mussels and cockleê to establish themselves in the chsnnels of the Outer Delta.
3.2. ENVIRONMENTALLY FRIENDLY EROSION CONTNOL The former sandbanks were autohatically tlansforrned into islands sfter the estuades vrere closed oÍl from the sea. Ilowev€r, thê shorelilreE of these islands were not stable, altal they eroded heavily. ln order to prevent fuÉher eroÊion by wind waves, various protection systehs were applied. 1 1 8
ln all cases €fforts. weÍe made to create a type of Protection €nabling the formation of an optiDal ecosystem. In some ca8es the island 1{ras given a recreational íunction_ For recreàtional shorclines difrerent requirements were e6tab-ltuhed, like safety lor chil&en, accessibility, etc. thê Drain forms of prctection werc small detached breakwaters (mainly consisting of gïavel), floating breakwaterc and direct shorc prctection with gravel beaches.
The conclusions of this research rrere that unprotecred shorelines in this area eroded by approximately 10 m/year. in the early years. The erosion speed decreases slowly to about 1 rtvyear, a value which is reached when the shorefacê plateau has a width of apprcximately 700 m. The depth of this plateau is of the order of 2.0 - 2.5 m. The maximum significant wave h€ight in this area is of the order of 1 m. There were no indications that the rcmaining erosion speed oí 1Íirear is decreasing any íurther. The prccess only stops when the island has entirely disappeàred.
As for conditions in these lakes, gravel beach€s were stabl€, br.rt they had ecological disadvantages, and were not acceptabl€ from a recreational point oí vieiv.
Detached bEakwaters were effectiv€ in these conditions if they were not farther from the main shoÍeline than 80 m. Behind a detachêd breakwater the remaidnq erosion was of tlte order ol a few crrtyear.
Fioating breakwaters were expensive and showed harally any efrect. Frcm arl ecological pojnt of view, detached breakwaters are very good, provided the distanc€ between the coast and the breakwater is 100 m or more. This is conhayy to the morphological requirements. Detached breakwate$ 'rith gravel on the lake side and sand on the shore side have advantages from an ecological point of view. This constructiod favours the feeding and brceding of bids.
3.3. EFFECTS OF BEACH NOUNISEITENTS
Beacb nourishmenr has Ewo aspects. Firstly, rhe sand has to be dredged somewhere and this drcdging has an influence on the environrdent. Secondly, the deposition of the sand atso has an inllueDce on the environment. The inÍluence of the tranEport of sand frorn the borrowing site to the beach can be neglected.
Placing the sand on a beach hardly aÍï€cts the biological environDent of the beach area, because of its high turbu_ lence. Only one or two species of v,/orms can be found on the beach. These worm populations recover quite fast after beach nourishmeÈt. The birds feeding oD f,nese worms are hardly inÍluenced by th€ noulishm€nt either.
IDvestigations of a nouriêhmeÀt at Schouwen (ftom 19g6) (Figue 4) showed that after one season the quality
of the biobass had recovered but that a change in the composition of speciDens had occunêd. However, it seems that this vsriatioE wa.s not caused by the nouriEhinent but was in fact a norrtral variatioa. lhe irÍluence of the nourishment on tlle morphological environment is in all cases very positive.
Ficruè 4 - NourÈhment Àt íhe Island of Schouwen in the southwest' èn pÊrt of the N€th€rlandg
Àt the bonowing site the situation is somêv,/hat diÍïerent. If sadd is borrov,.ed from deep water (20 m deep) and &edged with a suction hopper &edge, the influence on the morphological system can be neglected. Benthic fauna is compietely destroyed at thê bonowing site, but usually recovers rather quickly. Of course breeding area6 were not used as borrowing sites. Borrowing sand at such locationg could be expected to cause ecologtcal problems
For some noudshments, sand was borrowed from the outer deltas. Át those locations there is naturally a dynamic situation. If sand is borrorted corr€ctly, it will not have very much iníiuenc€ on the system, especially as the beaches themselv€s are also part of the same morphological system. Sand should therefore not be borrowed ftom isolÊted pits without tidal cufients. From an ecological point of view, borrowing sand froln a dynamic area does not present very much of a problem, because at those locations life is very well adapted to changes.
Borlowing sand flom lakes and ftom the Wadden area has hardly any inÍIuenc€ on the morphology of thé area itseif, apart hom th€ fact that isolated Pits wiII mostlv 611 up with fine mate al instead of sand From an ecological point of view, however, borrowing much sand from these sources is very often a problem At the Present time, borrowing sand fron ecologically valuable areas like Lake Grevelingen and the Eastern Scheldt is no longer allowed
3.{. STABILITY {AGAINST EROSIONI OF LARGE STRUCTIMES
Át the moment r soft " sea defences are favoured in the Netherlands. ÀI1 êÍposed construction such as the dredge spoit disposal alea near the Maásvlakte CIhe Slufter) is also Fotected in this way. The dike around it is not protected against erosion by stane cladding, bottom protection etc., but a beach is oeated in front of the dike. Due to wave and curetrt impact this beach will erode atrd must be regllarly nourished.
1'Lre shape of the Protêcting b€ach has been optimized using advanced mathematical eodels. Ttêse models will be discusEed in more detail ill Chapt€r 4.
EcorDmic aBalysis has ploved that coastal maintenance by periodical beach nourishment is cheapêr than defenBive structures like seawalls. This imPlies hovteve! that relatively high Baintenauce costs must be met. For SpverDment agencieÊ this may caul|e problem.s in the long term, because goverÍrment moíey is al$/ays êubject to political decisions of the moment.
3.5. EFFECTS ON ECOLOGY
Constructions iD ftont of existiBg dunes do alïect the ecology of the dunes, even if there is some distance between the construction and the (old dunes- Án idportant factor for the ecology of the dune area is the salt spray ftom the sea This spray is iDÍluenced by coDstructioDs.
The corutruction of " The Slufter " depot iu the êstuary area will bring about changes in the interactiod betÍ/een the dune area and the opea sea. Íhis inêraction findÊ pe.rticular expression in the salt particles which are spmyed iÈto the ai! above the sea and then blown ioto tbe dunes. Since the depot will paÉly protect the VoorDe coÊst from the sea winds, the quantity of salt particles carried into the dunês will defiease.
This is expected to lead to chsnges in condition-s for the vegetation in the outer row6 of drrnes. Plants which can tolerate salt now have an advartage over plants which cannot tolerdte it too well. A reduction in supply of sBlt Íollowing the construction of the depot will change thê vari€ties of plants growing in the dune area. The 6alt-toler-atidg valieties will be Iess widely distributcd and in smaller quantrtres.
In the outêr delta of the estuaÍine area in the south. western paÉ of the Netheriands, the Delta Project has caused many morphological changes. Some of them are indicated above. Recently a detailed iavestigation was made of the relatioÍs between the var{ous aspects of the area. Maps rvere drawn illustrating the wave clímate, tidal cur-rents, bottom topography, bottom composition (gxain size, silt content, heaw metal content), composition of benthos, occul-rence of flsh and birds. The investigation Íevealed a sigtlifi-cant correlation between some clssses oí benthic life and physical phenomena such Ês wave climate and curlent. Knowing these corÍeiatioÍÉ and kÀowing, from morphological studies (see Chapter 4 below) the charges to be expect€d in the bottom topography, predictions were made oí changes in marine life in this area.
The main conclusion from the invesiigation was that the chang€s occuring in this area are increasing its ecological value. lt has thercfore been decided not to int€rfere with these developmênts. RecÍeatíonal developmeÍtt of the outêr delta is not eÍrcouaged. No permits are graDted for making constluctions (like artificial islands for boating) in the outêr delta.
1. ADVANCED INI'ESTIGATION METHODS
4.I. GENERAL
Solving envircnmental or enginêering problems in the coastàl zone requires a set of investigstion tools, such as observation techBiques and models.
UEtil recently, Íield campaigns or operational monitoring systeEs were the only vray to gather quantitative data Íiom the coastal zone- Because of the difiicult logistics and the high costs involved, such data ate usually sparse in space, time and conditions. Consequently, synoptic studies on ihe efects of human int€rference in the coastal zone can harillv be ba8ed on these field data only.
Models of eome sort will thelefore be ne€ded for studies ol this tJDe, if not in the diagnostic phase (" how is tàe present system working ?,), then in the prognostic phase (,, what will be the effect of the proposed rtressure ? '). These models 6hould make optimun use of the availabl€ field data. In some c6ses, even hybrid prediction methods can be utilized, making use of models and phenoEenologicêl tech-niques at the same tiÍle.
4.2. MODELS FOR COASTAL INI'ESTIGATIONS
Roughly, th€re are two groups of coastal models, one desclibitrg physical aspects (tides, currents, í/aves, transport of Eatedal, oorphology), the other describi[g cherdical and biological aspects.
In prirrciple, these descdptiols are interconaected. In the case of coastal structures, however, the interrelation i6 áImost eEtirely one-way i ftottr morphological to chehical and biological factors.
The principal physical asp€cts in enviroÍmental iEpact studies of coastal Rtructures are mostly Eorphology atrd the tlamport of daterial. Models of these two classes of Dbeaom-enà víill now be outlined.
4.3. MORPHOLOGICAI, MODEIS
Numerical models of coa.stal morphology caa loughly be divided into the following four categories:
a. coasUine lrlodels (single-Iine, hulti-line), describing tàe cross-shore displacement oi one or more points of tbe coaatal profile over a given longshore sirelch;
b. coaEtal profile hodels, deÊdibing the deformation of the crosÊ-Êhore profile in the case of longBhore uDiforDity; c. duDe erosiotr hodels, de8clibiDg tàe storm profrle or the
dyaarnic evolution of dunes, beach and upper sholeíÀce, as8uDing longEhore uniformiiy;
d. coastÊl area bodel6, desctibing the morphological evolution in t{,o horizoltal dimeneione.
120
(a) COASTLINE MODELS
Singl€-line and double-line models are ilr opeÍàtional use in the Netherlands lPilarczyk and Van Overeem, 19871. The time eaalê of th€ morphological processes described by these models ranges fiom years to ceaturies. De Vroeg et al. de8c be a státe-of.thê-art application of a double.line model to the evolution of the Dutch cosst.
DISTANCE FFOM COÁST (d _ - INITTAL PBOFILE
- ? YEAFS --4 YE^FS
- 6 YEAFS _-- I YEAFS
Figrue 5 - Development of the coaÊtál Fotile at sone locationÊ Àlong the Dutah coalt (stêble, accetins, êrodiDs as obsêpêd), Êimulated with a CROSTRAN model ÍRoètvirk and Stivê, 19881
(b) COASTAT PROI1LE MODELS
Using cross-shore nrodels of waves, current6 and sedir'1etlt hanaport, the defonnatioD of the coastal Fofile is simulatcd oa a real tiEe basis (e.g. witlr the CROSTRAN-model; See Fig!Íe 5 and fRoelvink and Stíve, 1988D. A tjtrical time scale of the proces6e! desclibed is several years.
(c) DUNE EROSION MODELS
Or1ê cstegory of dune erosio! models uEes a s€Ei-empirical d€scription of the upper coastal prolile after a store. They are in op$atiodal use in thê Nêtherlands, ê.g. for security aasessEent of the dune-coast [TAW, 1984J.
g
i
Ê
I
I
oISÍÀNCE FRotl CoÁSï {n)
-A C C F E T I N G P R O F I L E
-E 9
I
436R. fon @*Rrds (nt
Fi$rrè 6 . Beaó scou in front of a dune rcvêtment, siEulated with '
a DIJROSTA model and measu€d in a lÀboÍaÈorv flume
lsieetz€I, r98?l
ADother cat€gory concerns models describing the dune and beach erosion process on a real titoe baBis, with a tt?ical tine scale of a few storms. T'he DUROSTA rdodol lsteetzel, 19871 Oigue 6) belong8 to this category.
íd) COASTAÍ, AREA MODELS
Coastal area modêls are Eeatrt to descíbe morphological evolutions in compiex situatiods vrithout a Pr€dominant orientation in the horizontal Plane. OÍïen they only give the residual sediment transpoÉ field and the attendant rate of erosion/deposition.
Compound model systems to describe this " initial trans' poÉ fietd arê in opelational u,se now (e.g. the COMOE system: See lBoer et al., 1984t. Ïlrey have P$ved their y'orth in impact assessment studies of coastal engineering works (harboÈ: entraoces, idets aÈd outlets, breakwaters, artiÍicial islands, etc.; seo also Íigure 7). A model of this t,?e was also applied iII the Slulter projêct, to optimize the shape of the ring dike from a maintenance point of view [Vàn Ordên, 19E9].
Fi8ute ? - Evotutior of thê Greveiins€n ebb-tidal deltá aftrr closu$ of the estuáIy, obsêíêd and hiDdcdtèd witl a COMOR nodel lsteijn et ar., 1989t
)=-j-t'-t-t--:
jí::;---'
è]=...
(ru**--$
C 2 > r A P - 2 , S n F = < N A P : 1 0 Í r /- )m dêoth contolriii..til .-.*
= : : : r * * "
= - : Í ! : , t : , D ' s , 2 1214,{. TRANSPORT II'IODELS
ApaÍ from the sand, the water trarsPorts other materials as well. such as:
- passive contaminánts, conveyed and disPeÉed by the water motion but rÀrithout any autonomous behaviour;
- silt. which is not only conveyed and dispersed but also stined and deposited (flocculation, settling), and
- mstter (chemicals, biota) that exhibits it8 own autonoÍrous behaviour (motion. reaction, decay).
A typical application of velsatile advectioídrffusion solver for these tranBpoÉ phenomeDa TDELWAQ; see Postma' 19841 is shown in Figure 8.
tisue 8 . ConceDtration oí N, P, Cd and HC ib the North Sea (situation
4.5. ECOLOGICAL MODELLING
A6 part of th€ chain ( structurê + water motroll + transport + water quality and biological conalitions + func' tions, biota ", traNport modelling is an inihsPen6able tool in the envilonmental iEpact asseÊsment of coadtal stÍuctures
The last step in the biological branch of this chain, the transformation of biological conahtionÊ into qoaDtities of biota, requrres dlescriptive ecologtcal model6. Given the en-vironmental conditions, these models describe the gro*nh, ali6placement atrd decay of a populatioD [Van Pagee et al., 19861.
1 r )
s. coNcLUSÍO\S
In th€ Nethèrlaíds, big works have been'undertaken and completed and thege continue to have an enormous ilíluence on the environment, That influence has been monitoled and evaluated. Bas€d on the experience gained and on integral scientific research, models have beed developed to quantify the environmental eflect of coastal wolks. For recent coastàl works these models ]vere used in order to determine which of the vaíous altemative pians had an acceptable impact on all a6pects oí the environment A coastal enÍiroirmental '
impact statement is presently requiled for all major coastal works. The tools to make such a statement are available
: silt€r 1980), conput€d with a DELWAQ model lPage et al' 19861
REFERENCES
-Bo€!, S., de VtiêDd, HJ. dd Wiad, H.G., 1984. A rnathematical model for tbe siEulation of morphological pro@ês in the coaltal arca. PÍ6.. 19th ICCE, Houston, Texas, p.1437'1453.
-De vloeg, JH, S!dt" ES.P and Balker' W.T.' l9SS coastal CenêÁis. PE. 21th ICCE, Malaga, SPain
. Huis ir 1 Veld, J.C., StuiP, J., Walther, AW. and Vt4tên, JM' va& eds. (r98?). The closule of tidal basi$; Delft Universitv Èess' 2d ed.
- Kohsiêk, L.H.M. (1988). Rereorkins oí fomêr ebb-tidat dêlta! into lalgr loatrhor€ bsr6 follorins the artiÍicial clo6uE of tidal inlsts in the southrêst of thê Netherknd!.
-- B{€r, P.L. €t .1. (eds). Tide--innuenced s€dimentáry envircMênt! and facies; D. Reidei publbhing compÀny, pp 113-122.
- Mulder, J.P.M., Spek, A.J.r,. van der, BerbèD, F.M.L. (1989) Closue of tidal ba!iD!; geonorphological consequencee ud environmertal iDpsct. To be published in Environnêntd C€olosy.
' Pilarczyk, K.W., Misdorp, R., L€ewis, R.J. and Vi$er, J. (1986). Strategy for êrdion contlol of Dutch êstuaries; 3rd l[t. Symp. on rirer s€dÍnenhtion; Jacksor, USA.
- Pilarczyk, K.W. ard Van Overesrn, I. (d) (198D. Àtanual oD aniÍicial beach trourbhment; CLiR PublÈhirg Foudatiotr, Gouda, The Netherlàtrds.
- Postma, L., 1986. A twodiEeBional water quaiity nodel apptication for Hons Kons Coastal Waters. Water Sci. ad Tecln, 16, no. 3/4, P.643-652.
'R@lviDk, J.A. atrd Stive, M.J.F., 1988. Wav6induced cÈs&shole flow vaiialion aad related ba! fomstion. To be published ir J. C€oph. ReB.
- St€etzel, H.J., 1987. A hodêl fo! beach áÀd dun€ pmnle change! near dune revetments. Proc. ASCE ConL . Coastal Sediments '87 ", New OrleaDs, Louisianê, p.87-97.
. SleijD, R.C., Van der Spek, A.J.F., I,ouleE, R. dd De V end, H.J., 1989. Nunerical nodel hindcast of the êbb-tidal delta evotution in ftont of the Dêltaworks. P.@. lni. Conf. "Hydr. and Env. Modetlins of Coaltal, Estuadne and River Watêrs ", Bladtord, U.X.
- TAW, (1984). Guidelines for thè ssesÊment of saísty of dures acting d sêá derens€. Enslish translation by CUR Publishins Foundàlion, Gouda. The Netherlands.
- Vad Orden, R.G.J.. 1986. Envtonmental impact statement on a pêninlula solution for the disposal of dr€ds€d natêlial. P!oc. XIth Wortd Dredsins Cotrcless, B shton, U.K., p.6?3-689.
. van Pagee, J.4., Gêrifsen, H. and Dê Ruijlèr, W.P.M., 1986. Tlabspori and vst€r quality nodelling ir rhê southern North Sea in reiation to coastal pollution r$each and control. Water Sci. ad Tech., 18, no. 4r5, p.245.256.
- \'erhagen. H.J. (1989). Coastat prot€ction dd dune ronagemèni in the Netherlandsj to he published in Jomát Df Codtal Res€a.ch
Part II
COASTAL STRUCTURtrS TO iIIITIGATE lIlE ENVIRON}IENTÀL I'}IPACT OF CONTAMINATED DREDGED MATERIAIS
r. INTRODUCTION
Given the great amount of silt to be annually d.redged in the Dutch harbour areas (3? dillion cubic mefes, with an extra of 10 million cubic metr€s frcm thê Western Scheldt Estuaryr it is elrdent that the disposal of the contaminated pan of iu calls for large.scale solutions.
2. THE PROBLEM OF DISPOSAL
OF CONTAMINATED DREDGED MATERIAL
IN THE NETHERLANDS
In this chapter a description will b€ given oí the way in rvhich the storage problem was solved lor the dredged
material originating fÍom the lower reaches of the River Rhine. Ttre gxeater part of the amount to b€ &edged in Dutch harbour átea6 comes from this region: 23 million cubic metres every year. The location of the largest port in the x,orld at the mouth of the same River Rhine, with its enormous economic interestg, is one of the reasons why the ( Policy Plan " with its detailed classificatiol system was drafted especially for this area.
Apart from the big depots for dredged mateÍia.l in the Netherlands : n The Slu.fter " and " The Pánot Beak ", a description will also be. given of other disposal areas in Germany, Belgrum and the Nethêrlanals.
2.I. SHOAI,ING PROBLEMS IN PORTS
NeaÍly all ports in the Netherlands have shoaling prob-lems. Constant &êdging in ports and navigation channels is ttecessary,
2.2. SOUE EXAMPLES OF DISPOSAL OF HARBOTJR SLUDGD
AII Port Authorities struggle with a lack of appropriate dumping aleas for contaminated sludge. The most important harbour areas in the Netherlands are :
(d EEMS HA.RBOUR DELFZIJL
Aurual quantity of maintenadce sludge to be dredged is about 1.5 mln m3. Conventional dumping areas are situated oÍfshore. Solutions for dumping about 0.5 mln m3 Class 4 sludge have been found in a special land location.
(b) AMSTERDAM , NORTH SEA CANAL AREA
Annual quantity of maintenance sludge to be dr€dged is about 3,5 mln mJ. Until 1985 the Port Àuthorities werc allowed to dump this quantity in open sea. Afterwards an exception was still made for contaminated sludg€ (in total about 0.5 mln m3) dumped in a former harbour, especially isolated.
(c) ROTTEBDAM
For this area reference is màde to Chapter 3 below.
(d) CENT-TERNEUZEN CANAL A.REA
Annual quantity of maintenance sludge to be dledged is rclatively small, till now dudped in the Western Scheldt Estuary. In a restlicted area, the contaEination of the cadal bottom turned out to be a more serious problem.
1 0
(e) WXSTERN SCHXLDT ESTUARY AIEA
ONCLUDING THE ANTWERP HARBOUR AREAI Annual quantity of maintenance sludge to be &edged is about 10 mln rn8, tiu now dumped in restricted (su.rface water) areás in the Western Scheldt Estuary. Contsmination of sludge urges the conshuction of apploPdate disposal areas, both estuarine and laud-bas€d (about one Íifth the size of the disposal area for the Rotterdsm Harbour area).
REFERENCES
Various policy d(nhênts (publicatior is in pro8re38).
2.3. POLICI PLAN FOR THE LOVER REGION OF THE RHINE DELTÀ
Until the nineteen seventies, alredg€d matelial was gener. ally used as filling material for lowlying areas (poldels). It then became clear that the major part of the silt was polluted with a vadety ol more or ]ess hazardous chemicals, heavT metals, etc. These pollutants oliginate from upstrcam and harbour linked industiies.
Às the disposal problem was most immediate for the lorr€r reaches of the River Rhine, with an annual amount of 23 million m3 to be &edged, a committee was formed to draft a plan i,, Policy Plan"t to solve the prcblem.
According to this plan, the real solution for the Ploblem was to reduce the dêgtee of pollution to such an extent, that the silt could be disposed of in the North Sea or usefully recycled. Hoerever, this could not be achieved at short notice. lt would take years befole the eilt would bê of such quality that it could be classified as (clean,,
2,4. CLASSIFICATION OF CONTAMINATED DNEDGED MÁTERIALS (a locsl clmsificltion !y6teln)
An ieportart par:t oí the " Policy Plan, was the claBËifi-cation of the silt, based on geographical grounds. Ïh€e categories or classes were distinguiÉhed, with a fourth non.geogÍaphic category for the most heavily polluted silt occuming locally in different parts of the regioB.
(a) CLASS 1: (13 mln m3 anuuallY)
(halauy contaminatêd / oí mariÍe origin), located in Rottêldam.Eulgpoort and the westêm paÉ of the RhiDe. Is being dumped into the North Sea.
(b) CLÀSS 2: (together with Class 3: 10 loln ma aanually) (ÊIightly coatarlináted) located in tàe Botlek area, Írest of the Ílouth of the Biver Oude Maa6. Fo. its disposal a tsedium form solution hàE to be found. S€e SectioD 3.1. 't24
(c) CLASS 3: (together with Class 2: 10 dln m3 aÍnually) (pollut€d in the riv€I and halbour basins located east of the River Oude Maas. For th€ disposal of this class of íraterial see S€ction 3.1.
(d) CLASS 4 ;
(heavily polluted) caused by locai chscharges of *'aste water into the harbour basins and r:ivers. The solution for the disposal of this class of mátelial is desclibed in Section 3.2.
At a lster stage, a more êoPhisticated classifrcation system, bÊsed olt deg]ee oí pollutiod (standards), should be designed in order to Eêke tàe system aPplicable to other legroDs iD the Netherlands as well.
2.5. SLT]DGE QUALITÏ
IA NATIONVIDE CLASSIFICATION SYSTEII}
(a) LEVÊLS OF SLUDGE QUALITY
Three Ievels of sludge quality (real qualitv standards) Base qua.lity = minimum quality level to strive for (han-sition measurements betweên Class 1 and Class 2);
Tesring values = quslity level used íor judging the free allocation of sludge (under certain conditions) or the undesir' ability ol the latter (transition measurements between Clássês 2 and 3);
SiglÍ.aliziJI,g value = indicational level Exceeahng thi6 ievel calls for examination to acceasory sanltary meàsuremefts (transitioD measurements between Clasees 3 and 4)
(b) RELATION BETWXEN DISPOSAI OF SLUDGE AND RDAÍ, QUALITY STAI\'DARDS
* 'Release idto sudacè waters', priDcipally permitted for dludge quality ieveh between 'Base quality' and 'Test' ing value'.
* 'CoBtrolled dispoeal' is permitted for sludge exceeihng tlte T€stiBg value'.
* 'Ieolated disposal' is requrred for sludge êxcêeding the 'Sigualizing values'.
3. SOLIJTIONS FOR HANDLING A}íD STORAGE
OF CONTAMINATED DREDGED MATERIALS
FON RHINE DELTA
3.1. TIIE SLUFTER
Accoldiflg to the abovêmetrtioned Policy Plan, a large' scale depot (150 mla m3, on tbe basis of the disposÉl of 2oo 0oo e3/week) for the 6tolage of Class 2 atrd 3 pollut€d
11
silt promi8es to be thê best wáy to solve the problêm for the next two decades.
All sorts of possible locations for such a large-scale depot were idvestigated, on land, off the coast aíd at sêa. After an Environhental lmpact Statement (EIS) lvan orded, 1986] the choice feII upon a site linked to the Maasvlalte, viz i ( The Slufte! " (Figure 9).
Fisue I - THX SLttFIEn: Larse-scalê d€pot for the stoÉse of contsminated dredged matê als on the Maalvlalte ned Rotterdam
The Slufter covers 260 hectares, is 28 metres deep, with an enclosing dam 23 metres above Mean Sea Level. The volume of the siit is reduced by consolidation. Due to the effect of the weight of the upper layers, the water is pressed out to the surface of the silt layer iDto the subsoil of the depot. This water has been in close contact with the polluted silt and is therefore also polluted.
Chemical reactions in the depot also determine the degïee of pollution of the vrater. A particularly important role is played in this respect by the breakdown of the oryanic matter in the silt. One result is the formation of gss. It is exp€cted that dÈing the storage period 10 7. of the space will be taken up by gas bubbles (methane and carbondiox-ide). Due to thê chemical reactions, the degre€ of pollution of the water will be variable during the firct 10 yeals. T'hereafter the situation will be more balanced.
After the total storage has been €ompleted, the quantity of water filtering thrcugh the bottom of the depot into the subsoii will gradually decrease and even rcach zero after a few hundred years.
The way in which the watêr draining out of the depot is distribut€d into the subsoil depends largeiy on the subsoil's geological structure and the difference between the relatively fiesh water iD the depot and the salt water in thê subsoil. This diíference will giv€ dse to a fresh water lens, floating on the brackish ground water (see Chapter 5 below). AII the wáier brought to the surlace in thb Iens disappea$ along the sides without spreading any fffther into the neighbour-ing subsoil. It is uncertain whether a fresh wat€r lens will
occur and coDtinue to eÍist under the deep part of the depot. Such a lens will certaiDly exist under the enclosing darn and the adjacent area, because of the surplus precipitatioÈ on this dam (400 Em/yeai).
The followiÀg conclusions can bê clrawn from various ground water distributioD pattêrn calculations :
(a) Most pollutioru occur ovêr a pedod of several thousanilÊ of years at a relatively high level in the ground wat€r at a lidited distance Èom the sludge in [he depot.
(b) Two pollutants (cadmium and zinc) will have moved noticeably turther during this period.
(c) In the immediate vicinity of the dam (50 metres) the presence oí the Íiesh v,tater lens tends to limit the rise in cadrnium and zinc concertrations to 10 to 20 % compar€d with the present natural levels.
(d) In the area lying fur:ther away, a further dse in the concentrations of cadlÏlium and zinc can be expected over a period of severai thousand years. A rise moreover, which stays rpithin the naturally occurring concentrations in ground water.
(e) The ground water in the nearby dunes will not be affected by polluted water draining ílom the depot. (0 No effects are expected on marine life on the sês bêd.
Because of the above conclusions measures such as sheeting or sealing layers in the depot were not taken into consideration. If things should turn out differently in prac-tice, a pumping system around the depot can be established to pump up the polluted water in the subsoil. By taking regular samples of the ground water, data can be assembled and used as a basis lor deciding whether the distribution of pollution actually taking place corresponds \rith irhat was predicted.
3.2. THE PARROT BETK
A special solution had to be found for the disposal of Class 4 silt. An'illvéirtory of the rnatedal in this category in the Lower Rhine Delta revealed that an amount of about 1.5 mitlion m3 had to be stored. As soon as the silt has b€en d.redged and stored, no drore silt of this category will be allowed to forh. Discharge permits arc linked rdth sFict legislation in this Íield. A plan was developed to creÀte a temporary site on a peninsula in the northerbmost part of the Maasvlakte. Because of its shape this site was given the name ( Parrot Beak
" Figure 10).
The site lies at a level of 5 m abov€ Mean Sea Level with dikes 3.5 m in height. Thus a depot of about 30 ha was cleat€d. Tbe slopes of the dikes ánd the bottom of the depot are covered $'ith " pla6tic
" (HDPE : High Densiry Poly Ethylene) sheeting, 2 mm thick, to prcvert the in{il. traiion of the highly polluted water into the subsoil.
This HDPE sheeting was one of the main conditions for getting legal permission to build the depot id this aree. In order to make the maJ.imum use oí the storage capacity, the silt will be pumped into the basin in thin layels.
12
Fisue 10. ï[m PARROT BEAK: Dbpoasl arca fo! heavily cortdi-nated dredsed matelial on the Maasvlalte Ded Rottêrdah
4. SOLIJTIONS
FOR HANDLING AND STORAGE
OF CONTAI}TINATED MATERIALS
IN THE FI]TTJRE
As shovm, the realization of the reservoirs ( The Slufter " and " The Parrot Beak " Fovides solutions for the storage ol respectively lightly and heavily polluted dredged material for the Roiterdam region for the rest of this century. Ho]\,ever, other la.rge-scale storage areas 6till have to bê considered for two maiÀ reasons:
'For other regions, such aa tlre southntestern region of the Netherlands (Antwerp, Western Scheldt EsÈuary), such sol-utions do not exist.
- ln the Netherlands at this moment a quantity of more than 100 mln toÀs of waste and dredged materials is generated annually. Ás it will take a considerable time (probably some decades) bêfoÍe the pollution is stopped at the source or reduced to an acceptable level, larg€ amounts of these materials still have to be Etored somewhere.
The pressure of the continuous Ílow of, iD psrticular, lightly contaminated matelials will demand large-scale sol' utions which can be quickly realized. In view of the scarcity of socially acceptable sites on land, fresh considelation should be given to the possibility of creating large-scale, well controlled and isolated areas in the sea itselí such ás : . disposal into pre-dredged holes in the sea bed which can be
cover€d ove! afte! completion of the disposal or, in other words, sealed ofi in order to grrarantee isolation, aDd - disposal in men-Írade island type storage baêin5.
In order to reconsider these po6sibilities, in November 1988 the Netherlands Royal Institute ol Engineers (K.I.v.I.) organized a s,'mposium in Rotterdam, where presentstion6 and contributions werê made by the various ministries coÍcerneal, by the Public Works Departmetrt of Rotterdam ard by environmeEtal and industrial (chehical and construc-tion industry) organizaconstruc-tiona.
126
With respect to the possibility of using man'made islarxd type res€roirs it was ahown that, for water deptàs of about 10 m, islands with a drameteï of át least 3 krn are economically visble, and, for water deptàs ol 20 m, islands with a diameter of 5 km or mole.
Further investigations into the Possibilities of such sol-utions will concentrate not so much on their conatruction possibilities, but Dore on the inÍluence such island schemes may have on coàstline develoPm€rt and th€ suroundiÀg s€a (bed).
5. STEP BY STEP INTEGRATION OF
CI\'IL AND ENVIRONMENTAL TECHNIQUES
S.I, GEMRAL CONSIDERATIONS
The basic desigrr principles of disposal sitPs have b€en dedved from conventional ísk aaalysis t€chniques, which requile consideration of the prcblem ftom three intercotrnec_ teal starting points :
(a) Á source of potentially dangerous chemicals.
(b) Á target in the surroundings in which the environment is considercd to be at Iisk.
(c) À pathway connecting the source to the target.
In practice it is usual to have to conside! multipie sources and tsrgets connected by several pathvtays- These i[clude :
(d Wind eÍosion of dry mate al and th€ transport of adsorbed or free phase contaminÊnts;
(b) Water erosion of material and the bansport of adsorbed and free phase contaminants, as well as the dispersion of any a3sóciated pore rtater contaÍrinants;
(c) Percolation of water through the depot and advection into the surrounding geo- aDd hydrospheles. Thi6 can also include ejectêd consolialation water;
(d) Direct uptale into the biosphere and the food chains. In thê loag-teÍm safety assessments, attention ako has to be paid to more remote failure scenarios,
Reliance can often be placed in natural (in 6itu) barrie$ ald in the long term these are the only orcs available. In the shoí term, it is possible to enhance the near-Íield barriers by civil etrgineering techniques, such as cut-oÍf Í.'alls, guard and scavengrng well points and capping.
PredictioD of the êíïects of the site desigtl oD the enviroDmental impact of the sita and of the most cost-.effective stlategleÊ for mitigating these eíïects togEther for!!
the basis of tÀe Dutch apploach to the design of diBpGal sites.
5.2. STORAGE FACILITY IN ZELZATE.BEI,GIUM
Chronologics.Uy, tàe ffst mojor facility to be designed ou theee IiDês was the landfill site to hold tàe cotrtsninat€d dredgêd Eatêrial froro the ship catral betwêen Ghert iD
Belgium and the Western Scheldt Estuary itr tàe Nêtberlanals. Thê site is located in a highly p€rmeable srnd aquifer itr the aglicultuel bufre! zoDe betlPeen two highly induskialized conurbations at ZeIzs.te ilr Bslgium' Tlre poteDtial fo! the Éigration of toxic cheÉicals ií the Àquifer war judged to be sevêre s.[d the almost noD-existent EÁtural barners wêre upgraded by addiÀg a bentonitecemêni curtaitr waII around the site, anchorêd in a deep clay stratum of low permeability and high integritY.
The migration pathways are such that in the short term the (polluted) transport and consolidation waters will be collected in a contlol ditch on the site' Beyond that time span, it was calculated that the permeability of the dredged matedal woulil be so love that further emissions into the surrounaliÍlgs, even after a signficant failur€ oí the cut'off wall, would be enviÍotrmentally acceptable'
5.3. STORAGE FACILITIES IN ROTïTRDAM AND HA]IIBIJRG REGIONS
The major facititv in thê Rotterdam área, " The ShÍïer D' has been descíbed above. Of importance lor thê safety analysis is tbe fact that the site is located oÍt top of a salt wateÍ aquife!, !íhich could allo*' migration oí the more mobile toxic components to areas of ecologicsl value within 2000 m of the site. However, it was not necessary to enhance the near-fielil natural barriers, reliance being Placed in the properties of the fresh water l€ns that would be created unaler the site by the pore water expelled in the initial consolidation phase. In the long têrm' i{hen this barrier will no loíger be available, the fluxes will have been reduced to such a level aE to pose no siglificant threat to th€ environmedt.
Finally, trvo sites have been considered where it has been though! necessary to reduce the potenttal for erosion and percolation to the absolute minimum' in both the long and the short ruÍ. These are the pesticide_contadinated sediment site )n one of the RoLterdam harbours and a proposed site in the Waalaienzee, a major nature reserve, to hold the highly contaminated dredged materiai ftom the Hamburg port area' ln both tbese cases it has been decided m excavate deep into the river- or s€a bed, to la lill the matelial in questior itt the pit so forfted, atrd to cap to seá bed level with an unpoiluted barner material. In the forme! case capping wás effected imdediately aíXer fiIliDg and in the Iatter case capping wiII be improved by the long'term collapse of the su.rrounding dikes over the site by witld and wave action. This Iast site is illustrated in Figure 1l'
Fi8ue 11 -Co!Êidersd sitê for deP@it of dledged InÀtlriáI iÀtó the sea bed
h tàe long term, the emission scedarios froE both sites will be difrrsion and rêdox_dolainatêd, with very Iow emission l€vels. In the short term' cate laust be tsken to avoial majo! eDis6ions and Figurê 12 illuskatês the expeci€d coacedtlatioo aleveloPment in the surrounding sea bed et tbe IlaÍrburg siÉ, again for cadmium The conceDtratioDs sholÀ'n are of the order of thê cur:Tent backSrou[d levêIs and are to be tskeD as worst-case eallmalea.
-timê
(years).---figue l2'Expected c6dmiu concedration developmedt in tbê sur' roundinc sea bed at the Hambug site
6. CONCLUSIONS
Given a lot of effort and money it is Possible to soive the storage problems for contaminated dredged material for the short and medium telm. The cases discussed in this part of the presentation demonstrate the importánce of an inte' grated approach to the environmental and civil enginee ng desigin tasks illvolved.
I{owever, the disposal of large quantities of contÀminated mate al rcsults in a concentration of significant amounts of toxic anal potentially harmful chemicals in a relatively small area. Large areas of land anavor water are concern€d and to alate all efforts to solve the problem of stodng dredged material still fail to bdng the ultimate solution nearei'
For the long term, eÍIorts have to be mêde to limit the dis€ha.rges of polluted waste water into rivers atrd harbour basins in the N€thertands as w€ll as in the neighbouring count es.
ln the Netherlands, technical and legal studies are being carrieal out in order to solve the Foblem before the necessity arises for building a second Slufter or Parrot Beak' The Municipality oí Rotterdam is discussing the pÍoblems i{ith inalust es, water quality contloilers and other municiPal authorities along the River Rhine
PaÍt ol the l€gal studv will be concerned with the quesiion as to whether the Municipal Authority of RotteÍdam can charge th€ additional silt disposal costs due to the pollutants to one or rnole of the industries causing this pollution and whether it can institute cívil prccedures io achieve this.
I o,1s
I = - 0.10 .9 ; ru uru) I I 0 , 0 0 12't1 4
The aim of the technical study is to identify the sources from where the various pollutants are being discharged and to calculate what contribution each individual discharger mak€s to the total pollution ol the silt.
In the opinion of the Rotterdam authoritiês, all these efforts should frnally result in the dver silt being of such a quality that it can be transported to the sea without any prcblem, employed again - as it was in the past , to raise the level of polders, or put to 6ome óther good use.
REFERÊNCES
(a) VÀn léÉuwen, P., Kleirbioêsêm, W.C.H. and Cr@newesen, H.J. (1983): A policy plan for the disposal of dredged material from the Port of Rotterdam in . Papers prêsentêd at the World Dredgiag Concress 1983 in Singápore
", pp.509-527.
(b) Stigter, C.: Consideratios from private industry rcgarding lárge-scalê stolag€ of &edged materials, Synposium StoÉge Bulk Wáste ànd Dredged Materials. Netherldds Royal Institute oí Engineers (K.l.v.I.), Rott€rdam, the Netherlands, 30 Nov€mber 1988.
RESUME
Au couÍs des dernières décennies, plusieurc ouwages d'ingénierie cótière ont eie realises aux Pays,Bas, dánE l€ but de protéger Ie littorai et d'améliorer la protection contrê les inondations, pour améliorer les routes d€ navigation maritime et pour créer des bassins d eau douce sssurant une réserve suflisante en eau de bonne qualité.
Le principal problème rencontré lors du développement et de la création de ces Fojets était Ia conception d'ou.vrages capables de résister aux élémeíts (environnement), répondant aux objectifs précités au plus intéressant prix de revient.
CependaDt, au moment même oÈ ces projets de grande enverglre (Projet Delta, Port en eau profonde de Rotterdam) étaie[t en cours de développement et de réaiisation, d'autres phénomènes apparurent, tels que Ia crcissance explosive de I'industrie en général €t du bassin du Rhin en particulier. De plua, I'augmentation de ia production couplée à l'amélio-ration généralfuée du niveau de vie se traduisaient par une sollicitation beaucoup pluÊ élevée par personde des ressources natuell€s (telles que I'eau, les produik agÍicoles, l'éconoEie etc.) et par l'augïnentation de la production de produits de déchet.
Les Pays-bas, situés à I'embouchure du bassin du nhin, très peuplé et iDdustrialise, et engagés alans l'execution de
plans d'aménagement à $ande échelle dans des zones cótières et des estuaires du delta du Rhin oÈ I'équilihe écologique est délicat, étaient un des prcmierc pays à être conftontés à la nécessité de concevoir des o[yrages capables non seulement de résister aux éléments (envinnn€rnent), mais aussi de protéger I'envircnnement lui-même d'inter-fércnces humaines trop radicales, En d'autres termes, le développement de deltas et de plaines c6tières devait se faÍe non seulement en fonction de cdtères purement techtdques, mais auÊsi en t€nant compte de beaucoup d'autr€s aspects.
La prise de conscience grandissante de ce processus a mené à la modiÍication du Prcjet Delta au cours de son exécutioÍr. Ainsi le bras de mer de l'Escaut Oriêntal lra pas été fermé complèt€metrt, hais reste en jonction ouvertê avec la mer. Une pode marée-tempête n'est fermée que lorsqu'il y a danger d'inondation.
Ce qui Fécède n'est que le contexte du présent rapport comportant deu.x parties :
1. L'impact écologique des ouvrages cótierc en général, 2. Ouwages cótiers destinés à stténuer l'impact sul le milieu
de produits de áragage contailinés.
La première partie contient une brève descdption d€ quelques grands projets de génie civil en cours aux Pays-Bas. L'accent est mis sur les conséqu€nces de ces projets qui n'appartiennent pas au domaine de I'ingénierie et sur l€s rnesures prises pour y fairc íace. AÍin d'aboutir à une apFoche multidisciplinaire intégt'ée, des modèl€s numé ques avancés, décrits dans le rapport, ont été developpés.
Dans Ie cadre de cette philosophie nouvelle, Ies solutions " douces ", tels que I'ensablement des plages, sont à prélérer aux ouwages dgides. II est aussi tenu compte de solurions lavorables au milieu pour la protection des rivages.
La deuxième partie du rapport traite des solutions, déjà réalisées ou à I'étude, à la contamination des fonds des bassins portuaires et des voies navigables. Après une descdp. tion dê Ia classification des prcduits de ilragage contaminés, suit une descdption de sites de déversement poul matfuiaux contaminés déjà réalisés ou à l'étude en Belgique, dans la République Fédérale d'Allemagrre et aux Pays'Bas. Une attention spéciale est accordée à quelques solutions réalisées aux Pays-Bas pour l'entreposage Íespectif de produits de alragage peu et fortement pollués.
L'iDtention et l'espoir des auteurs est de rnonher avec ce lapport, à l'aide d€ projets déjà réalisés, comment Ies régions c6tières peuvent êtle gérées emcacernent et utilernent à partir d'une approche multidisciplinaire intégrée des problè-mes soulevés.
